CCN1 Combustion and Ventilation — Questions and Answers
Question 1: What are the three elements required for combustion to take place, commonly known as the 'fire triangle'?
- Gas, electricity, and water
- Fuel, oxygen, and ignition (heat) (Correct answer)
- Fuel, nitrogen, and pressure
- Gas, air, and a flue
Correct answer: Fuel, oxygen, and ignition (heat)
The fire triangle consists of fuel, oxygen (from the air supply), and a source of ignition (heat). All three must be present simultaneously for combustion to occur. Removing any one element will extinguish the flame or prevent ignition.
Question 2: What is the ideal CO₂ reading for natural gas when an appliance is correctly set up for complete combustion?
- 5-6%
- 7-8%
- 9-10% (Correct answer)
- 12-14%
Correct answer: 9-10%
For natural gas (methane), the ideal CO₂ reading is approximately 9-10% when combustion is complete and the appliance is correctly adjusted. This indicates the correct air-to-gas ratio is being achieved. A CO₂ reading significantly below this range suggests excess air, while above it may indicate incomplete combustion.
Question 3: What is the minimum free area of ventilation required for an open-flued gas appliance with a rated input over 7 kW in a room?
- 5 cm² per kW of rated input above 7 kW (Correct answer)
- 5 cm² for every kW of rated input
- 10 cm² per kW of rated input above 7 kW
- No additional ventilation is required
Correct answer: 5 cm² per kW of rated input above 7 kW
For open-flued appliances with a rated input exceeding 7 kW, additional ventilation of 5 cm² of free area per kW above 7 kW is required. The first 7 kW is considered to be served by normal air infiltration in the room. This is specified in BS 5440-2 and Approved Document J.
Question 4: What dangerous gas is produced as a result of incomplete combustion of natural gas?
- Carbon dioxide (CO₂)
- Nitrogen dioxide (NO₂)
- Carbon monoxide (CO) (Correct answer)
- Sulphur dioxide (SO₂)
Correct answer: Carbon monoxide (CO)
Carbon monoxide (CO) is a highly toxic, colourless, and odourless gas produced when natural gas undergoes incomplete combustion due to insufficient oxygen supply. CO poisoning can be fatal. Signs of incomplete combustion include yellow or orange flames, sooting, and increased CO/CO₂ ratios.
Question 5: A room-sealed gas appliance draws its combustion air from where?
- From the room in which it is installed
- Directly from outside the building through a duct or balanced flue (Correct answer)
- From an adjacent room via an air vent
- From the loft space above the room
Correct answer: Directly from outside the building through a duct or balanced flue
A room-sealed (Type C) appliance draws all its combustion air directly from outside the building through a purpose-designed duct or the outer part of a balanced flue terminal. This means the combustion circuit is completely sealed from the room, making it safer as it does not deplete oxygen levels in the occupied space.
Question 6: What is the approximate air-to-gas ratio required for the complete combustion of natural gas?
- 5:1
- 10:1 (Correct answer)
- 15:1
- 20:1
Correct answer: 10:1
Natural gas requires approximately 10 volumes of air to 1 volume of gas for complete (stoichiometric) combustion. In practice, slightly more air is provided (excess air) to ensure complete combustion, typically giving an actual ratio of around 11-12:1 at the burner.
CCN1 Combustion and Ventilation
What are the three elements required for combustion to take place, commonly known as the 'fire triangle'?